Direct drive rotary valves are known for controlling flow of hydraulic fluid between a high pressure fluid source (P) and fluid-powered load, such as a hydraulic actuator. FIG. 1 schematically illustrates a known system in which a four-way rotary valve is used to meter hydraulic fluid to and from opposed chambers of a fluid-powered load in the form of a hydraulic actuator. A common rotary valve configuration includes a spool rotatably mounted within a bushing. The spool and bushing include various ports and passages configured to open and close flow paths to and from the fluid-powered load depending upon the rotational position of the spool relative to the bushing. For example, where the load is a hydraulic actuator having first and second opposed chambers, the rotary valve may be configured to open a first fluid supply path (P to C1) for delivering fluid from a pressure source to the first chamber, while simultaneously opening a first fluid return path (C2 to R1) by which fluid may return from the second chamber to a system reservoir. The same rotary valve may be adjusted by rotating the spool to open a second fluid supply path (P to C2) for delivering fluid from the pressure source to the second chamber, while simultaneously opening a second fluid return path (C1 to R2) by which fluid may return from the first chamber to the reservoir. When the first fluid supply path (P to C1) and the first fluid return path (C2 to R1) are open, the second fluid supply path (P to C2) and the second fluid return path (C1 to R2) are closed, and vice versa.
The spool may be driven by a torque motor through a limited range of rotation relative to the bushing. The spool has a rotational null position relative to the bushing, wherein flow of pressurized fluid is shut off to both chambers. Rotation of the spool relative to the bushing away from the null position progressively opens supply and return paths. Rotation of the spool in one direction from the null position supplies fluid to the first chamber via first supply path (P to C1) and enables return flow from the second chamber via first return path (C2 to R1), whereas rotation of the spool in an opposite direction from the null position supplies fluid to the second chamber via second supply path (P to C2) and enables return flow from the first chamber via second return path (C1 to R2).
A problem encountered in rotary valves of the type described above is that the inflow of pressurized fluid acts on spool surfaces, causing imbalance in the spool and making the spool more difficult to rotate. As a result, torque motor load requirements are increased, accompanied by greater size and weight of the motor. For many applications, particularly aerospace actuation systems, the added size and weight may be unacceptable.
Efforts have been made to overcome the problem of imbalance. Under one approach, the spool and bushing are provided with redundant metering edge pairs arranged to balance forces imparted to the spool by the flow of pressurized fluid. However, designs according to this approach have been very costly to produce due to added complexity in machining additional ports and flow channels in the spool, which typically is a miniature component on the order of 7 mm in diameter by 10 mm in axial length. Moreover, this approach presents added challenges with respect to synchronization of the edge pairs. Under another approach, a large radial clearance is provided between the spool and bushing, with the spool being mounted on bearings. However, designs according to this approach have done a poor job of controlling internal leakage. Consequently, designs using one or both of the mentioned approaches have not been widely adopted by industry.
What is needed is a rotary valve design that is geometrically simple for ease of manufacturing, yet also reduces spool imbalance to an acceptable level.